Most artificial satellites, spacecraft and other propelled devices such as aircraft, ship and ground vehicles (collectively referred to herein as vehicles) require information about their locations and/or attitudes to accomplish their missions. This information may be obtained from one or more sources, such as the global positioning system (GPS), ground-based radar tracking stations and/or an on-board star tracker.
A star tracker is an optical device that measures bearing(s) to one or more stars, as viewed from a vehicle. A star tracker typically includes a star catalog that lists bright navigational stars and information about their locations in the sky, sufficient to calculate a location of a vehicle in space, given bearings to several of the stars. A conventional star tracker includes a lens that projects an image of a star onto a photocell, or that projects an image of one or more stars onto a light-sensitive sensor array (digital camera).
One type of star tracker is “strapped-down,” meaning its view angle, relative to its vehicle, is fixed. Another type of star tracker can be aimed mechanically, such as in a direction in which a navigational star is expected to be seen. Using data from the photocell or sensor array, the star catalog and information about the star tracker's view angle, relative to the vehicle, the star tracker calculates a position of the vehicle in space.
Strapped-down star trackers are mechanically simpler than mechanically aimable star trackers. However, the fixed view angle of a strapped-down star tracker limits the number of navigational stars that may be used. Mechanically aimable start trackers can use a larger number of navigational stars. However, aiming a prior art star tracker, relative to its vehicle, with the required precision poses substantial problems. In either case, preventing stray light, such as from the sun or reflected from the moon, reaching the photocell or sensor array is challenging, particularly when a navigational star of interest is apparently close to one of these very bright objects.